If she is getting poor grades, etc, telling that she improve her grades will not work at this age/stage of life. You need to provide step by step approach.
- Talk to her to identify any problems. If she denies having any problems, you point them out to her.
- Make a plan to remedy the problem. Most of the times, kid will need consistent and sustained practice in the given area. If reading is a problem, make sure to work on reading for 15 mins everyday (or every weekday).
- There will be plenty of resistance. You can get over it by the point system for rewards. In my house it was - you put full effort on these 3 well defined exercises and you can play video games for half an hour. Do not overwhelm the kid - the work should take no more than 15 mins at her full effort. For the first few weeks the kid may take a couple of hours to do it, but keep at it with patience. If she does not do it, the only thing she looses is the immediate reward.
- Once she consistently does her work, slowly her grades will improve. At the end of the reporting period, you can point out how her hard work has shown results.
- She will slowly develop good work habits. For my kid, it took about 2 years - I still have to tell him what to work on each day, but he asks for it, and does it willingly.
One thing to note here is that the reward/consequence is never after getting the grade report. The grade report only informs what to focus on next. The reward/consequence is to motivate the daily consistent practice that is needed for improvement.
Some good parenting books may help you. I personally liked:
- Kids Are Worth It! : Giving Your Child The Gift Of Inner Discipline
by Barbara Coloroso
- How to Talk so Kids Will Listen...And Listen So Kids Will Talk
by Adele Faber, Elaine Mazlish
Others may suggest other books. My approach would be to read the books, and pick the parts which you think will work in your life. All parental figures should be on-board for the plan. Also, plan should be flexible enough to avoid power struggles, but not too flexible that everyday work is skipped. You with her father should figure out the balance.
And yes, as pointed out, taking all toys away is pretty harsh and random. Always discuss with kid and give the kid a chance to recover before punishing. For example, you may have a rule - homework first, play later. You need to tell this to the kid a lot of times before she starts playing. If she still starts playing, then you separate her from the toys and put the toys away from reach. All this is enormous amount of overhead for you, but she is at the right age to guide her towards good work habits - old enough to understand, but not a teen where one just rebels.
You also need to build a healthy relationship with her for any of your discipline to work. A large percent (say 95%) of your interactions should be happy and positive for you (any parental figure) to be able to discipline the kid.
Kids will be kids, pick your battles.